A pressure-measuring plug of the above-mentioned type is known from U.S. Pat. No. 7,228,730 and US 2005/0061063.
In these known pressure-measuring plugs, the heating rod is displaceable in a sliding manner in the axial direction over the full length of the plug body. With a pressure-measuring glow plug of the above mentioned type, the combustion pressure is transmitted directly via the heating rod to a pressure sensor.
The known pressure-measuring plugs have a resonance frequency at about 8-9 kHz depending on the length of the glow rod. This limits the bandwidth of output of the pressure sensor to 3-4 kHz.
Advanced combustion strategies for gasoline and diesel engines in the future depend on the existence of accurate pressure feedback from each combustion cylinder during the entire engine cycle (compression-combustion-exhaust cycle). These strategies may or may not include Homogeneous Charge Compression Ignition HCCI combustion, and can result in high pressure release rates which require a fast and accurate pressure response. Fast pressure sensors are in this context sensors with a high bandwidth response and a low response time. There is a need to measure the pressure in a combustion cylinder continuously and real time.
In gasoline engines, the ‘knocking’ of the engine, i.e. self-ignition, can be seen in the course of the pressure in the combustion chamber at a frequency around 7 kHz. In the frequency spectrum also harmonic frequencies, e.g. 14 kHz, of the ‘knocking’ frequency can be found. In view of this, a pressure-measuring plug with a higher bandwidth then the know plugs is needed.
Furthermore, in the known pressure-measuring plugs, the pressure-sensor and corresponding electronics to measure and condition the signal obtained from the piezoresistive elements are located in a housing mounted on top of an open end of the plug body. After insertion of the pressure-measuring plug in the engine, the housing is outside the engine. Therefore, the temperature of the electronics will correspond to the temperature outside the engine. In some applications, the pressure-measuring plug is located near a turbocharger or other heat source of the combustion engine. The turbocharger will increase the environmental temperature up to around 200° C. The regular temperature change from outside temperature, which is normally in the range of 0-30° C., to a temperature of 200° C. while driving, will stress the electronics and increase the failure rate.
Furthermore, the maximum pressure to be measured will increase. As a result, the mounting force on the housing will increase. The mounting forces on the housing will in the known pressure measuring plugs provided with rod, result in an offset in the sensor signal, requiring that the sensor electronics have to compensate for this after mounting the plug in an engine. EP0793082A1 discloses a pressure measuring plug with a cylindrical measuring body inserted into a the housing and fixed at an opening of the housing by welding.